The present invention relates to new methods permitting the preparation of high purity aluminas and the alumina products obtained thereby, and more particularly, it relates to the production of such aluminas starting with ammonium alum.
High purity aluminas are commercially prepared by the thermal decomposition of ammonium alum. The preparation of alumina starting from ammonium alum proceeds through a series of complex reactions: dehydration of the alum, thermal dissociation of the anhydrous alum leading to the aluminum sulfate, and finally, thermal decomposition of this latter.
When the alum is heated without special safeguards, it begins by melting in its own water of crystallization. Then, the solution evaporates, which leads to its progressive thickening. When saturation is attained, the product crystallizes. In the course of this phase, some of the vapors are entrapped in the solid which is formed.
This promotes a considerable expansion in volume which ultimately results in the formation of an extremely porous and light mass which has the appearance of a meringue. The anhydrous alum then decomposes by steps, but the apparent volume of the solid is not seen to change greatly.
Ordinarily, the alum is placed in crucibles, generally of silica, in which all of these steps are sequentially carried out. The thermal treatment is effected either in intermittent furnaces or in continuous circulation furnaces of the type used for firing ceramic articles. However, the decomposition velocity of the aluminum sulfate is slow under these conditions. It eventuates that a prolonged residence time is necessary to conduct the preparation to its end. For all these reasons, the furnace productivity is extremely small and the equipment for a given production capacity entails substantial expense.
A few other solutions have been proposed in the prior art. British Pat. 514,538 describes a process in which the starting material is atomized in a first stage of thermal treatment in the form of a solution, a suspension, or a molten bath containing water, through a calcination column by means of air or steam and in which the anhydrous salts formed are then passed, in a second stage of thermal treatment, in that case after comminuting, (in suspension) in a current of hot gases at high temperature in a decomposition chamber. Such a process is very costly and of long duration, and thus hardly invites use on a commercial scale.
It is also known from German patent application 2,215,594 to inject alums containing the water, by means of compressed air, directly into a burner flame having a temperature of 1200.degree. to 1600.degree. C. and thermally decomposing some of these in such fashion. Nevertheless, this process also itself is commercially costly, and it moreover presents the inconvenience of being very subject to unstable operation.
According to West German Pat. No. 2,419,544, alum in powdered form is heated by means of hot gases and the powder is compacted into brick form prior to its entry into the second stage of thermal treatment. These prior art processes present multiple difficulties: the residence times are very long or it is difficult to control large-scale apparatus, which necessitates consequent low production; the product is frequently contaminated in the course of the treatment; instabilities in the desired operating conditions are frequent; and it is sometimes even recommended that recourse to a separate briquetting operation is necessary.
The .gamma.- and .eta.-aluminas, also called "transition aluminas", obtained starting from ammonium alum by the commercial prior art processes have the following characteristics:
Residual sulfur, about 0.2 percent PA1 Specific surface, less than 125 m.sup.2 /g PA1 Loose or bulk density (d.sub.app), about 0.15 g/cm.sup.3
The .alpha.-alumina, or corundum, obtained by calcination of these transition aluminas has a bulk density not exceeding 0.2 g/cm.sup.3.
The .gamma.- and .eta.-aluminas (transition aluminas) have numerous commercial uses. Notable uses are as catalyst supports, starting materials for single-crystal manufacture, as fillers for plastic materials, for metal polishing, and in obtaining porous membranes. The transition aluminas also find a commercial utility as the starting material for producing .alpha.-alumina, or corundum.
For certain applications, as for example in catalysts, it is desirable to have transition aluminas with a higher specific surface, that is to say, equal to or greater than about 200 m.sup.2 /g.
For other uses, for example the preparation of corundum, it is desirable to have transition aluminas with a higher bulk density than those aluminas prepared from ammonium alum according to the industrial processes of the prior art, which prior art materials have densities of about 0.15 g/cm.sup.3. Corundum is utilized notably for the manufacture of high pressure sodium vapor lamp tubes, the preparation of aluminates in manufacturing phosphors, the fabrication of cutting tools for metal-working, and the fabrication of bioceramics.
For most of these applications, the .alpha.-alumina ought to be formed by a compression before sintering. In these cases, if the piece ought to have well-defined dimensions, it is desirable that the shrinkage on sintering be the smallest possible. Such shrinkage is in the same proportion smaller as the density of the raw alumina is higher.
The prior art commercial processes have not permitted the manufacture of high purity .alpha.-alumina, that is to say, derived from ammonium alum, having a bulk density greater than 0.2 g/cm.sup.3. But, for the very wide diversity of uses of these aluminas, correspondingly there is a great variety of considerations concerning their physico-chemical properties of density, specific surface, corundum content, and so on.
There accordingly exists a need to develop industrially useful processes for preparing high purity aluminas starting from ammonium alum offering a very great flexibility, notably with regard to the possibility of producing a great variety of alumina physico-chemical characteristics.
There equally exists a commercial need to develop a process for the preparation of high purity aluminas starting from ammonium alum which will be economical and reliable.